This invention relates to optical fibre networks.
Optical fibre networks currently available present a considerable number of problems which are not encountered in point to point systems. For example, there are difficulties in introducing new nodes (terminals) and removing old ones without the system being disabled; the number of users is seriously limited by the losses of the nodes or couplers, unless they are active, in which case there are network reliability problems and the nodes are expensive; special receiver designs are required to enable a large dynamic range of received signal levels to be handled, and high capacity per user is difficult to achieve since in a packetised system only modest capacity is available to each user despite the necessity for high bit rate transmission in the network itself.
The growing interest in local area networks (LANs) has raised a new component need, that is for a cheap and convenient means of connecting and disconnecting terminals within multiterminal fibre networks. Generally the techniques proposed so far require the fibre to be broken to insert the terminal. This inevitably means that the network is disabled whilst the change is made. It is of course possible at the time the network is originally installed to splice-in an optical coupler or to introduce an electro-optic terminal at a location where the need for a future terminal is anticipated. However, this is both restrictive and expensive in a system with unpredictable terminal locations. It is also very inefficient in the use of available power. Ideally the installation cost of a multi-user network should be dominated by the number of users at that moment in time, rather than the ultimate capacity. The cheaper the system on the day of installation the better.